Many growth/differentiation factor receptors are composed of an intracellular kinase domain and an extracellular ligand-binding domain. These receptors are classified into some families based on the similarity of domain-structures.
Recent genomic information has revealed many novel genes that are considered to encode receptors based on their structures. These genes are referred to as orphan receptors since their ligands are unidentified. However, from the information about the function of ligands for known family receptors and specificity of expression sites of these orphan receptors, it is expected that some of their ligands are important physiological factors for controlling biological functions.
Insulin receptor-related receptor (hereafter, also referred to as “IRR”) is one of the orphan receptors belonging to an insulin receptor family (Shier et al., J. Biol. Chem., 264: 14605, 1989). Known insulin family members such as insulin and insulin-like growth factor (IGF)-1 bind to their corresponding receptors as ligands and regulate functions such as growth of cells expressing the receptors, expression of glucose transporter and suppression of the hepatic gluconeogenesis pathway via auto-phosphorylation of the receptors. Therefore, it is expected that growth/differentiation and functions of IRR expressing cells can be regulated by using an IRR ligand via binding to IRR and phosphorylation of IRR. However, there has been reported that known insulin family members such as insulin and IGF-1 cannot bind to IRR nor induce signal transduction for phosphorylation of IRR (Zhang et al., J. Biol. Chem., 267: 18320, 1992). Thus, the identity of a ligand specifically binding to IRR has remained unknown.
Increased insulin resistance such as obesity attributable to changes in life style is a major cause of the recent increase in the number of type-2 diabetes patients. On the other hand, it has also been reported that diabetes does not develop when compensatory growth/differentiation of the β cells, which are insulin secreting cells existing in the islets of Langerhans in the pancreas, occurs in response to insulin resistance (Terauchi et al., J. Clin. Invest., 99: 861, 1997). Although a physiological mechanism for regulating mass of the pancreatic β cells is unknown, elucidation of its regulatory factor will provide novel preventive and therapeutic methods that increase or maintain the endogenous or transplanted pancreatic β cell mass in a diabetes patient.
In recent years, based on findings in knockout mice lacking IRS-2, which is one of intracellular substrates of insulin receptor kinase, it has been found that IRS-2 is essential to increase of the pancreatic β cell mass corresponding to an amount of insulin demand. Since then, involvement of the insulin receptor family has been strongly suggested (Withers et al., Nature, 391: 900, 1998). On the other hand, it has been confirmed that IRR is highly expressed in the pancreatic β cells. Further, it has also been confirmed by using an artificially prepared insulin receptor (IR)/IRR chimeric receptor, that IRR kinase phosphorylates IRS-2 (Hirayama et al., Diabetes, 48: 1237, 1999). And thus it is estimated that an unknown ligand binding to IRR (IRR ligand) acts on the pancreatic β cells as a factor for inducing growth, differentiation and survival of the cells. Therefore, means and a method for regulating growth/differentiation of pancreatic β cells will be provided by obtaining the IRR ligand or an agonist thereof, and it is also expected that it serves as a therapeutic agent for diabetes via regulation of growth/differentiation of the pancreatic β cells when it is administered to an living body.
Moreover, since it has been reported that IRR is expressed in specific localized tissues and cells such as nerves, kidney and stomach (Reinhardt et al., Endocrinology, 133: 3, 1993; Watt et al., Adv. Exp. Med. Biol., 343: 125, 1993), IRR is also considered to be involved in growth/differentiation of these cells. Therefore, it is also expected that, if the IRR ligand or an agonist thereof is used to induce growth/differentiation of cells in these tissues, it will also be useful as a therapeutic agent for congenital or acquired neuropathy, renal disorder, gastrointestinal injury due to medication, autoimmunity, inflammation or the like.
Further, it is expected that an antagonist of the IRR ligand can be a therapeutic agent for diseases attributable to or accompanied with hyperplasia or hyperactivity of the aforementioned tissues or cells, for example, proliferative nephritis.
As described above, however, the IRR ligand has not been identified and there is no method for conveniently searching for its agonist or antagonist since the IRR ligand is unidentified.
Meanwhile, epithelins/granulins are known as factors for inhibiting growth of tumor cells (Shoyab et al., Proc. Natl. Acad. Sci. USA, 87: 7912, 1990) or as cysteine-rich proteins existing in leukocyte granules (Bateman et al., Biochem. Biophys. Res. Commun., 173: 1161, 1990). As epithelins/granulins, there have been known seven kinds of them in total including granulin A (epithelin 1), B (epithelin 2), and C to G.
As functions and uses of polypeptides belonging to the epithelins/granulins, there have been described a tumor cell growth inhibiting action, epithelial cell growth accelerating action (acceleration of wound healing) and keratinocyte growth accelerating action (treatment of psoriasis with an antagonist) of epithelins 1 and 2 (Shoyab et al., Proc. Natl. Acad. Sci. USA, 87:7912, 1990; WO91/15510A). There has also been reported that the epithelin/granulin precursors themselves (PCDGF, GP88) accelerate growth of tumor cells and fibroblasts (Zhou et al., J. Biol. Chem., 268: 10863, 1993; Xu et al., J. Biol. Chem., 273: 20078, 1998; WO 98/52607). Further, there have been suggested that epithelin 1 binds to a 140-145 kDa protein on a human breast cancer cell membrane (Culouscou et al., J. Biol. Chem., 268: 10458, 1993) and that PCDGF binds to a 120 kDa protein on the membrane of a mink lung epithelial cell strain (Xia et al., Biochem. Biophys. Res. Commun., 245: 539, 1998). However, proteins to which these epithelin 1 and PCDGF bind have not been identified. Thus, receptors of epithelins/granulins still remain unknown, and there has been no report suggesting their association with IRR.